Along with the continued and increasing use of video displays has been the problem of the man/machine interface. Traditionally, control of displayed information or cursers has been via the use of a keyboard. More recently, however, a number of devices allow an operator to directly interact with a video display. These devices have included light pens, desk-type mouse controllers, or touch input devices, such as switch matrixes or opto-electronic matrices. While switch-type overlays, placed adjacent to a video display, are generally inexpensive to apply and to utilize, they are also susceptible to contact wear as well as the distortion of the video information which is presented to the viewer or operator, particularly in situations involving a high degree of use. Opto-electronic matrix schemes utilizing light which is generally in the infrared region, however, create a matrix which is invisible to the viewer or operator and, therefore, do not distort the video information displayed and is not subject to wear in high use environments. Systems utilizing opto-matrix frames are well known in the art. An example of such a system includes U.S. Pat. No. 4,267,443, entitled "Photoelectric Input Apparatus," issued May 12, 1981, to Carroll, et al.
Attendant to the use of such systems are problems such as increasing frame resolution without a corresponding increase in the number of components used, surrounding or ambient light compensation, and the optimization of emitter/detector driving and detecting networks. Such systems, however, still have certain drawbacks. For example, the use of a large number of components results in higher costs. Additionally, some systems exhibit difficulty in compensating for reflection or glare, which sometimes results in the failure of the system to detect a touch or hit.
It is generally desirable to have a touch entry system which minimizes the number of components necessary for addressing and sampling light emitters and light detectors. It is also desirable that the device dynamically compensate for ambient light effects and for variations in emitter light output and detector sensitivity. At the same time, however, the device should employ a minimal number of components. Accordingly, it is an object of the present invention to provide a touch entry system and circuitry for such a system which minimizes the number of components necessary for accurately addressing and sampling light detectors and light emitters surrounding an irradiated field.
Other ambient light compensation systems have required the use of large amounts of energy to drive light emitting devices, such as infrared light emitting diodes (LEDs), in order to ensure that the intensity of the light emitted by the LEDs is greater than the intensity of the ambient light. Such an approach requires the use of expensive light emitting and light detecting elements. Such an approach also requires that even high performance LEDs and phototransistors be operated outside normal operating ranges.
It is an object of the invention to provide a touch entry system and circuitry for such a system which utilizes a feedback loop which selectively holds the signal resulting from the ambient contribution at a pre-selected reference level to provide increased accuracy and confidence in the determination of the presence of an element in an irradiated field.
It is a still further object of the invention to provide a practical touch entry system and circuitry for such a system comprising a substantially rectangular frame with optical emitters and detectors disposed in the sides of the frame with a device for sequentially sampling and activating the light detectors and light emitters.
A still further object of the invention is to provide a cost effective touch entry system and circuitry for such a system to overcome ambient light problems in a touch entry system by utilizing and detecting light pulses.
It is a still further object of the invention to provide a touch entry system and circuitry for such a system which compensates for ambient light effects which is easily manufactured and produced with a minimum number of components and at a minimal cost.
U.S. Pat. No. 4,243,879 discloses a touch panel with ambient light sampling. This device includes sampling means for sampling the ambient output level of each of a plurality of light detectors. Storage means are employed to temporarily hold the signal representative of ambient light. Comparison is then made of the signal received from the same detector illuminated by a corresponding source of infrared light to detect the additional light. If no additional light is detected then a hit due to the presence of an opaque element blocking the infrared beam is detected.
The device employed in U.S. Pat. No. 4,243,879 does however have certain disadvantages which are not characteristic of the instant invention. It is difficult to conduct an accurate sampling with this prior art device. High precision resistors must be employed to subtract the ambient light signal from the activated signal. Therefore there are limits to the range of ambient light which can be handled because of the offsets and the errors in the subtraction make it easy to run up against a limit on the amount of ambient light which can be handled. With the instant invention, a servo loop is used to zero out the ambient. This servo loop is attached around the whole amplifier stage, all offsets and circuit resistor or capacitors type errors can be zeroed out. That allows compensation for all factors in a circuit in addition to ambient light. Therefore the amplifiers can be used at a much higher gain level than was possible with the prior art. This allows the front end to be set at a much lower gain, thereby giving a wider dynamic range to the ambient signal. The lower the gain on the front end, the more ambient light that can be handled. Signal swings at the front end can then be compensated by the servo loop containing a programmable amplifier according to the preferred embodiment of the invention. Previously sampling of the ambient and the activated signal was done before gain was applied and that required a comparison of two different resistor values in the prior art device disclosed in U.S. Pat. No. 4,243,879. Therefore any errors in the resistor value were exaggarated by the subsequent amplification of the signal. In the instant invention the baseline signal corresponding to the ambient signal is at ground which is where it should be for the analogue to digital converters employed in the touch input controller which comprises the preferred embodiment of this invention.